Core porosity analysis and apparatus therefor



H. J. WELGE March 29, 1949.

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C Y L V252 E 6 m R Y 5 Patented Mar. 29, 194@ CORE POROSITY ANALYSIS ANDAPPARATUS THEREF Henry J. Welle, Tulsa, Okla., assigner to Standard OilDevelopment Company, a corporation of Delaware Application August 20,1946, Serial No. 691,840

The present invention is directed to a method and apparatus for studyingcores obtained from a subterranean formation.

The principal object of the present invention is the provision of amethod and apparatus for the study of porous rock-like materialsobtained as cores from subterranean formations or from any other sourceto ascertain certain of their properties, such as their retentivity fordifferent fluids and productivity of different fluids and the like. In atypical case, a method and apparatus are employed for the study of coresof rock-like materials secured from oil bearing formations to determine,among other things, the productivity of the formation under water drive,the per cent of contained oil retained by the formation as residual oilafter production by water drive at a given pressure, and the per cent ofwater retainable by the formation as connate water when the water wassubjected to the displacing action of accumulating oil in a petroleumreservoir.

An additional object of the present invention is the provision of amethod and apparatus of the type described characterized by simplicityand ease of construction and operation. An additional object is theprovision of an apparatus of the character described which is easilyportable so that it can be used at the well site, if desired.

Further objects and advantages of the present invention will appear fromthe following detailed description of the accompanying drawing in which,

Fig. 1 is a plan view, partly in section, of one type of apparatusaccording to the present invention;

Fig. 2 is a pair of curves which represent results obtainable by thepractice of the method of the present invention;

Fig. 3 is a verticai section of another form of apparatus according tothe present invention; and

Fig. 4 is a vertical section of stili another modification of theapparatus of the present invention.

Referring to the drawing in detail. numeral I designates a block,preferably of transparent material, such as Lucite" or similartransparent resin, having an interior recess 2 which is generallyfrustro-conical in configuration. Seated In the recess is a stopper 3 ofNeoprene or other oil-resistant material having a central cylindricalpassage (I. Arranged in the passage I is a cylinder 5I! of the porousrock-like material to be examined, held in place at its upper end by adisc l of carbon or other preferentially oil-wettable material and atits lower end by a disc t of fritted 4 cui-ns. (ci. 'rstsrv glass orother preferentially water-wettable material.

The stopper 3 is held on its seat .by a threaded plug 1 of Lucite" orsimilar transparent resin which is screwed into the upper end of therecess 2. This plug has a central passage 8 which is connected totransparent /tubing 9 by a coupling member I0 which may [also be atransparent resinous material. l Between the inner end of the plug andthe disc 5 is afspring Il which serves to hold the parts firmly inplace.

'I'he lower end of the stopper and the disc 6 rest on an annular seatprovided by a sleeve I2 which constitutes an extension of lower plug I3;which is likewise of transparent resinous material and is screwedinto/the lower part of the recess 2. Lower plug I3 has] a centralpassage Il which is connected to a U-shaped tube I5 by a suitablecoupling I6, both of which are likewise preferably made of transparentmaterial. It is desirabley that both tubes 9 ahd I5 be calibrated forvolume, so that changes' of liquid level in them may be translated intothe volume of liquid displaced.

In using the device for determination of connate water, the rock-likematerial and the disc i are rst soaked with water as by being boiled inwater so as to insure the complete filling of their pores with water.The disc 5 may in similar manner be saturated with oil. The rock sample50 is vthen arranged in place in the manner shown. With the U-tube andthe space beneath the disc 6 completely filled with water, then oil,such as crude oil, is forced through tube 9. The oil will pass throughthe disc 5 but will not pass through the disc 6. As pressure is appliedto the oil, water leaves the sample 50 and enters the space below thedisc 6, causing the water to rise in the leg of the U-tube. In this waythe maximum replacement of water by oil at any given pressure may bereadily noted by observing the rise of water level in the open leg ofthe U-tu'oe at the selected oil pressure. The pore space of the samplehaving previously been determined by other conventional methods, thequantity of water contained therein when saturated can be ascertained.Therefore, by difference, the quantity of water remaining in the coresample under oil drive at a given pressure may be calculated.

It is in general found that all the water cannot be replaced by theabove means. By the term connate water is denoted the water that is notreplaceable from the core by pressure alone. It

Y is thought that the connate water is trapped in b1ind" or alternatecapillaries, and its replacement by oil thus prevented. However, it isnot acume 3 desired' to make the claims :or the present in. ventioncontingent on the correctness of this 'explanation for the phenomenon.

In the operation of this device, care should be taken to select a disc Iwhich will not pass oil under the operating pressure. The minimumpressure that must be applied to cause the oil to pass through thecapillaries in the water-wetted disc is called the entry pressure, P, ofthe disc medium. It is generally related to the enective pore radius, r,of the medium by the relation,

In the following table are given typical values calculated with the aidof the above equation, when c=15 dynes per centimeter, a representativevalue for a crude oil-water interface:

The result of a typical experiment is shown in Fig. 2. The full curve i1represents the replacement of water by oil; it is particularly to benoted that, after the initial large replacement without much change inpressure, a great increase in pressure causes only slight additionalreplacement. At the maximum pressure reached, about 12 lbs/sq. in., thefluid content of the core is considered comparable with its fluidcontent in the underground petroleum reservoir before the reservoir wastrapped by drilling.

Up to this point the use of the apparatus described in the presentinvention bears some similarity to the method of operation described inUnited States Patent No. 2,330,721, entitled, "Method of determiningconnate water content of cores. One point of difference, however, isthat the present invention proposes crude oil as the displacing phaseinstead of compressed air. The displacement by crude oil simulatesunderground conditions more precisely. The use o! oil is also necessaryby the processes disclosed hereinafter, which are yaltogether outsidethe scope o! United States Patent No. 2,330,721, and which constitutefurther points of difference between it and the present invention.

After any desired quantity of water has been expelled from the core, thedirection of fluid flow may be reversed by removing the pressure fromthe oil and applying pressure to the water phase. 'I'he dashed curve i8shown in Fig. 2 represents typical behavior when this is done, in amanner analogous to the production of crude oil by rise of theunderlying water table. The displacement of oil by water may again befollowed readily by observation of the change in level of the oil or ofthe water, since the water, in turn, cannot penetrate the upperoil-wetted carbon disc.

In practice it is usually found that iluid replacement is aided byinsertion of a somewhat compressible contacting medium between eitherretaining disc and the core. For the lower. or water-wetted disc, sheetsof soft tissue paper moistened with water may be inserted. or a layer ofa paste of clay or oi' barium sulfate in water. For the upper carbondisc, sheets of paper have been treated with water-repellent materials,such as silicones. 'They are inserted after moistening with oil.Alternatively, a layer of carbon black, moistened with oil, may be usedas the contacting medium between the disc and the core.

A gas also may be used to displace either water or oil from porousmedia, using suitable arrangements of discs and contacting media obviousto one skilled in the art. It is, in general, found advantageous to soarrange the apparatus that the denser fluid, i. e., the water or theoil, occupies the lower part of the apparatus, the gas occupying theupper part.

In carrying out this portion of the present invention, it is desirableto use a gas possessing only limited solubility in the oil or aqueousphases.

such as nitrogen or argon. Otherwise the volume oi' the liquid phasesdisplaced may be increased by the dissolved gas, and hence volumetricreadings of displacement may be in error.

The processes described above may be used to ascertain the fraction ofcrude oil in an underground deposit that may be recovered throughdisplacement by either an aqueous phase or by a selected gas. Knowledgeof this fraction is of great practical importance in petroleumproduction technology, inasmuch as it complements knowledge of the totalamount of oil lying in a deposit, as obtained from information on thetotal void space and the proportion of the latter occupied by theconnate water.

In Fig. 3 is shown a somewhat simplified form of apparatus which is notoi' the reversible type as illustrated in Fia. 1. In this ligure,numeral .Il designates a transparent glass cylinder having fused in itsbottom a fritted glass disc 20 on which rests a sheet 2i of compressiblefilter medium upon which is placed a sample core 22 of the rocklikematerial to be studied. The upper part of the cylinder is provided witha Neoprene stopper 23 between which and the sample is a spring 2l forholding the sample ilrmly upon its bed. The -stopper 23 is provided withan inlet tube 2B for the introduction of the driving medium.

The cylinder l! constitutes the upper part of one leg of a U-tube 20,the open leg of which is arranged in front oi' a scale 21 such as apaper scale marked oil.' in squares, in order that the change in liquidlevel may be ascertained. and hence the volume of liquid displaced.

Inusing this arrangement for the determination of replaceable water andconnate water, the compressible medium 2| is a soft absorbent tissueYsuch as Kleenex paper. The sample is prepared as in the case of Fig. 1,and with the parts in the position shown, oil is forced in through tube25 under nitrogen pressure. 'I'he amount of water displaced isdetermined by reading the level in the open leg of the U-tube. Asindicated in the drawing brine may be used in place of water in thisstudy. A seal of red oil or similar petroleum oil may be used in theU-tube to minimize volumetric changes caused by vapor ization of thewater from the U-tube.

It it is desired to use this apparatus to determine replaceable oilunder gas drive, the disc 2l and the medium 2| are preferably bothtreated to render them preferentially wetted by oil as by being coatedwith a silicone or a material such as lead oleate. In this case the coremay be a natural oil-containing core or a core which has beenartificially saturated with oil to the point of saturation existing inthe formation being investigated. The U-tube in this case is filled withoil and, with the parts in the position shown, nitrogen is forced inthrough the tube to any desired pressure and the increase in level ofthe oil in the `open leg of the U-tube is observed.

In Fig. 4 is illustrated a particular modification of apparatus used fordetermining the oil produceable under water drive. In this case thecylinder designated by numeral 28 is inverted with the fritted glassdisc 29, suitably coated with a silicone or other substance torender itpreferentially oil-wettable, arranged near the upper end. The sample 30to be studied is held against the compressible filter medium 3| arrangedon the lower surface of the disc by a spring 32 which is held undercompression by a Neoprene stopper 33 sealing. the lower end of thecylinder.

The upper end of the'cylinder terminates in an upright tube 34 arrangedin front of a scale 35 and provided with a stop cock 36. The scale 35may suitably be a sale made of paper ruled o ff in squares. The stopper33 is provided with a passage to accommodate a tube 31 through whichwater is forced to provide the water drive. The stopper is also providedwith a depression to sample resting on one side of a porous disc capableof preferentially passing said fluid, maintaining a column of said fluidin contact with the other side of said disc and feeding to said confinedspace a second fluid immiscible with said first fluid and not passableby said disc until a predetermined pressure is built up in said confinedspace and measuring the amount of said first named fluid displaced fromsaid sample.

2. A method for determining the retentivity of a porous rock-likematerial for water and its productivity of oil under water drive, whichcomprises coniining a sample of said material saturated with water in aclosed space between and in contact with a pair of porous discs, one ofsaid discs being capable of preferentially passing oil and the othersaid disc being capable of preferentially passing water, maintaining acolumn of water in contact with said latter disc on the side thereofopposite the sample and maintaining a column of oil in contact with saidformer disc on the side thereof opposite the sample, applying pressureto said oil column to a predetermined value, observing the increase inlevel in said Water column as a measure of the amount of water displacedfrom said sample, thereafter applying a pressure to said water column,and observing y the increase in level of said oil column as ameasfacilitate the introduction of a syringe 38 or similar suctiondevice so disposed that its suction end is adjacent the juncture of thelower face of the disc and the cylinder. The purpose of this syringe isto suck out of the system any gas which may be trapped therein whenywater is introduced into the system so as to insure that the systembelow the disc is completely full of water.

In the use of this device the sample is arranged as shown, having beensaturated with oil as-described above and illustrated inFigl 3, a spaceabove the disc up to a point in the measuring tube is filled with oil,andthe system below the disc is filled'with water, the syringe beingemployed during the filling with water to insure elimination of air.Then pressure is applied to the water tube and the increase in level ofthe oil in tube 34 is measured. By adjusting the pressure in thewater'tube the amount ofoil` recoverable at any given pressure may beascertained. As indicated in Figure 4, brine may be used in the systemin place of water.

In all cases above, it may be found advantageous to check the amount ofany given liquid displaced by physical or chemical `analysis of therock-like sample at the end of a displacement.. Analysis `for the wateror the oil is, in general, carried out by methods familiar to oneskilled in the art of chemical practice.

The nature and objects of the present invention having thus been setforth anda specific illustrative embodiment of the same given, what isclaimed and desired to be secured by Letters Paten is:

1. A method for determining the retentivity of a porous rock-likematerial for ai fluid which cornprises connning a sample o! saidmaterial satuo rated'with said fluid in a closed space with said ure ofthe oil displaced from said sample.

3. An apparatus for the study of cores comprising a transparent block,having a central passage, a pair of porous discs maintained in saidpassage, in position to receive a core sample between their inner faces,one of said discs being preferentially permeable to water and the othersaid disc being preferentially permeable to oil, a stopper for each endof said passage, a tube adapted to hold water communicating through oneof said Stoppers with the outer face of said disc which ispreferentially permeable to water and a tube adapted to hold oilcommunicating through the other of said stoppers with the outer face ofsaid disc which is adapted preferentially to pass oil.

4. An apparatus for determining the productivity of an oil-containingformation under water drive, which comprises a chamber, a porous discforming a partition in said chamber, said porous disc beingpreferentially permeable tooil, means for pressing a sample of theforma- REFERENCES CITED The following references are of record in the leof this patent:

UNITED STATES PATENTS Number Name Dato 2,327,642 Horner Aug. 24, 19432,330,721

Leverett Sept.' 28. 1943

